Cryogenic pumps use the inherent physical force of dispersion to remove gas atoms or molecules from active circulation within a system. The dispersion force causes neutral gas atoms or molecules striking a sufficiently cooled surface to bond with the surface, removing them from the environment surrounding the cooled surface for a mean residence time before the atoms or molecules break free. When a gas atom or molecule sticks to a cooled surface in this fashion, it is said to be "adsorbed." If the mean residence time during which it is adsorbed is sufficiently long to serve the purposes of a cryogenic pump, the molecule is said to be "pumped."
By pumping atoms and molecules and thus removing them from the surrounding environment, cryogenic pumps can be used to create vacuums which can be used in vacuum systems ranging from general purpose vacuum systems to ultra high vacuum systems. Cryogenic pumps are used in a wide variety of applications, including: particle accelerators, thin film deposition, evaporation applications, ion implantation, and the creation of simulated space environments.
The duration of the mean residence time for any particular gas upon the sorbent is dependent upon the temperature: the cooler the sorbent, the longer the residence time. Further, certain gases can only be adsorbed at extremely low temperatures. Accordingly, efficient cryogenic pumping systems operate at extremely low temperatures such as 70 Kelvin or 15 Kelvin.
The amount of gas which a particular cryogenic pump can effectively remove from a system is limited. A cooled surface acting as a sorbent has a limited number of adsorption sites. The process of adsorption is continuous, as previously-adsorbed atoms and molecules break away from the sorbent and are replaced by newly adsorbed atoms and molecules. Early in the adsorption process, many adsorption sites are open, and the rate at which atoms and molecules are adsorbed greatly exceeds the rate at which they break away from the sorbent. Later in the adsorption process, few unused adsorption sites remain, and rate at which atoms and molecules break free from the sorbent becomes approximately equal to the rate of adsorption. At this point, the sorbent is said to be saturated.
Once the sorbent is saturated, no further net gains in pumping can occur until the already-pumped gases are desorbed from the sorbent. Traditionally, to enable further pumping, the sorbent is removed from the pump and heated until the adsorbed molecules are effectively "baked" out of the sorbent in a process called regeneration. Following regeneration, the sorbent is recooled and cryopumping is again possible. For each regeneration the sorbent must be removed from the pumping system and baked; this adds time and complexity to the pumping process. The capacity and thermal conductivity of the sorbent are the primary characteristics controlling this "bakeout" time.
Accordingly, the choice of a sorbent material for a cryopump is critical. Some cryopumps such as cryopanels and Meissner coils adsorb gases on their cooled outer metallic surfaces. As metals are not highly effective sorbents, these pumps' ability to pump gases are non-optimal for the temperatures achieved.
State of the art sorbent choices for cryopumps include coconut charcoal and synthetic zeolites, which have become standard sorbents for cryogenic vacuum pumping applications. Synthetic zeolites are used, for example, in liquid refrigerant cryosorption pumps and are formed in pellets usually one to two millimeters in diameter. These pellets are disposed within a pump volume with point-to-point contact between the pellets and the walls of the pump chamber. Coconut charcoal is used, for example, in compressed helium cryogenic pumps. It is produced by burning coconut husk and is formed in small, randomly-shaped chunks which are usually bonded to a nickel plated copper substrate with a thermally conductive epoxy adhesive. Both are desirable sorbents because of their relatively high surface areas.
However, the random shape of coconut charcoal and the pellet shape of synthetic zeolites reduce their effectiveness as sorbents. Because of their thickness, adsorption sites in the inner cores of these sorbents remain largely inaccessible to atoms or molecules in the pumping environment even after the outer surface of these sorbents are fully saturated. Adsorption sites in the inner core are used only when atoms or molecules previously adsorbed on the outer surface of the sorbent work inward upon desorption. It can take numerous adsorption-desorption steps for an atom or molecule to reach adsorption sites in the inner core. Overall, this reduces the effective surface area per volume of these sorbents, reducing the capacity per volume of the cryopump.
The shape of these sorbents also limits the degree of thermal conductivity achievable between the pumping mechanism and the sorbents. Synthetic zeolite, which is a ceramic, naturally has poor thermal conductivity. The time for thermal energy to pass between the zeolite pellets by point-to-point contact is extensive. For the coconut charcoal, the thermal bond between the nickel-plated copper and the sorbent is poor due to the irregular geometry of the coconut and the limited thermal conductivity of the epoxy bonding agent. The epoxy used with coconut charcoal further limits the temperatures at which the pump may be heated during regeneration, as typical epoxies will soften at high temperatures.
The poor thermal conductivity to these sorbents increases the amount of time necessary both to cool the sorbents initially during pumping, and to bake the sorbent to desorb the accumulated gases and regenerate the pump. One such baking/cooling cycle alone may take multiple hours. Particularly in applications requiring a large number of cryopumps, such as linear accelerators, the extended bakeout times increase operating costs, making it necessary to include many redundant cryopumps to ensure continuous pumping, and require large numbers of personnel to oversee the regeneration of the pumps.
Additionally, these sorbents, particularly synthetic zeolites, are friable and produce dust which can be swept into a clean vacuum system during the turbulent flow that occurs at the onset of evacuation of the system. This can contaminate ultra-high vacuum experiments, and cause purge values to malfunction. Coconut charcoal and synthetic zeolites are not used in simpler cryopumping devices such as cryopanels and Meissner coils both for this reason, and because of the relatively high degree of effort necessary to attach a multitude of sorbent particles to the outer surface of the device.
It is an object of the current invention to provide a sorbent for a cryopump which has a large pumping capacity and also offers improved thermal conductivity and flexibility of design.
It is a further object of the current invention to provide a sorbent for a cryopump which can be resistively heated to produce rapid desorption of gases and thus rapid regeneration of a cryopump.
Another object of the current invention is to provide a sorbent for a cryopump which is non-friable and minimizes the risk of dust corruption of the vacuum produced by the cryopump.
Other objects and advantages of the current invention will become apparent when the carbon aerogel sorbent of the current invention is considered in conjunction with the accompanying drawings, specification, and claims.